The present invention relates generally to electric heaters and more specifically to positive temperature coefficient (PTC) electric heaters.
Two well-known types of electric heaters are the resistance-wire heater and the positive temperature coefficient (PTC) heater. In a resistance-wire heater, heat is produced by passing current through an electric wire. In a PTC heater, heat is produced by passing current through a plurality of electrically interconnected PTC (ceramic)pellets. The primary drawback of the resistance-wire heater is its propensity for overheating, which can often result in a fire if there are flammable items within the heater's immediate vicinity. More specifically, the heat source in a resistance-wire heater can often reach temperatures as high as 1600 degrees fahrenheit, well above the limit of combustible materials. Without installing a an effective safety mechanism in the resistance-wire heaters, the possibility of overheating and fire is present.
In contrast to the resistance-wire type heaters, the possibility of overheating and fire in a PTC heater is not present. This is a result of the self-limiting characteristic of the PTC element's temperature. Due to the physical chemistry of the doped ceramic material of which the PTC elements are made in a PTC heater, the PTC elements effectively shut off at a relatively low temperature. Typically, once a PTC element's temperature rises to approximately 340 degrees fahrenheit, the resistance of the element rises to a point to completely eliminate the flow of current through it, thereby preventing any further rising of the PTC element's temperature. This physical characteristic of a ceramic PTC heater element prevents overheating and eliminates the possibility of fire.
In addition, PTC heaters are more durable than resistance-wire heaters. A limitation of PTC heaters, however, is that the heat produced is directed over a relatively small area. This is due primarily to the size and shape of the heater assembly containing the ceramic pellets.
Some pertinent patents include U.S. Pat. Nos. 5,245,692, 5,197,112, 4,965,432, 4,737,616, 4,703,153, 4,518,847, and 4,459,466.